1. Field of the Invention
This invention relates to programmable buffers which are made on a single integrated circuit chip and a programming method for such buffers. Buffers are data storage devices generally used for data transfer, to which data elements can be written in from one source and read out to another. Whether or not programmable, buffers made on a single chip are referred to as buffer chips; such buffers communicate with the external environment through I/O pins of the buffer chips in which they are embodied. Generally, a buffer chip includes designated buffer status pins for indicating certain monitored buffer status conditions, such as full, empty, half-full, etc. By programmable, it is meant that the particular buffer conditions which are monitored and indicated to the external environment at certain buffer status pins are changeable by external selection; the procedure for entering program information is referred to as programming. Entering program information into the buffer chip programs the buffer chip. Programmability allows a user of the buffer chip to change the buffer status conditions that are monitored, according to the particular requirement of the application environment in which the buffer chip is configured. Without the programming feature, the monitored buffer status conditions are not changeable, which limits the usefulness of the buffer chip.
2. Description of the Prior Art
Programmable buffer chips are known in the prior art. Programmability is not itself the subject of this invention. Rather, this invention is directed to a particular method of programming which eliminates a major disadvantage common to all programmable buffer chips in the prior art, and to a buffer chip which is programmable by such a method. That major disadvantage of programmable buffer chips belonging to the prior art is the problem of compatibility, i.e., backward compatibility with buffer chips which are not programmable.
Compatibility in the context of the present invention means interchangeability. A programmable buffer chip is compatible to a non-programmable buffer chip if the non-programmable buffer chip can be removed from its position in an original circuit and replaced by the programmable buffer chip, pin for pin, without causing a malfunction in the original circuit. The original circuit would continue to operate as it otherwise would with the basic non-programmable buffer chip. Compatibility in that sense is backward, as the more advanced buffer chip having the programming feature is made compatible with the basic buffer chip not having the programming feature. Nevertheless, compatibility has never been achieved. No prior programmable buffer chip is known which is compatible with a nonprogrammable basic buffer chip having the same number of functional I/O pins.
The use of dedicated programming pins in the prior art is one reason why compatibility cannot be achieved in the past. A dedicated programming pin is one at which the signals applied thereto relate exclusively to programming of the buffer chip. Where there is such a dedicated programming pin in a programmable buffer chip, when the programmable buffer chip is substituted for the basic non-programmable buffer chip, a number of conflicts may arise. There may be an incongruity in the number of pins, or there may be a situation in which the original system not designed for operating with a programmable buffer chip may send signals to the dedicated programming pin. In either case, the system would not obtain a proper response from the substituted buffer chip.